How to get chloride ions into the cell

For the first time, a molecular movie has captured in detail the process of an anion transported across the cell membrane by a light-fueled protein pump. Publishing in Science, the researchers utilized the unique synergy of a Free Electron Laser (SwissFEL) and synchrotron light source (SLS) offered by PSI to unravel the mystery of how light energy initiates the pumping process − and how nature made sure there is no anion leakage back outside.

Many bacteria and unicellular algae have light-driven pumps in their cell membranes: proteins that change shape when exposed to photons such that they can transport charged atoms in or out of the cell. Thanks to these pumps, their unicellular owners can adjust to the environment’s pH value or salinity.

One such bacteria is Nonlabens marinus, first discovered in 2012 in the Pacific Ocean. Among others, it possesses a rhodopsin protein in its cell membrane which transports chloride anions from outside the cell to its inside. Just like in the human eye, a retinal molecule bound to the protein isomerizes when exposed to light. This isomerization starts the pumping process. Researchers now gained detailed insight into how the chloride pump in Nonlabens marinus works.

The study was led by Przemyslaw Nogly, once a postdoc at PSI and now an Ambizione Fellow and Group Leader at ETH Zürich, in close collaboration with the ALVRA team at SwissFEL and the MX team at the SLS. It is one of the first studies to fully combine experimental capabilities at these large-scale research facilities, bridging the gap in time resolution to record a full molecular movie of a protein at work. Slower dynamics in the millisecond-range were investigated via time-resolved serial crystallography at SLS while faster, up to picosecond, events were captured at SwissFEL – then both sets of data were put together.

Read more the PSI website

Image: Photoactive chloride pumping through the cell membrane captured by time-resolved serial crystallography: Chloride ions (green spheres) are transported across the cell membrane by the NmHR chloride pump (pink).

Credit: Guillaume Gotthard, Sandra Mous